The US will send 560 additional troops to Baghdad to assist Iraqi forces in recapturing Mosul later this year.
US Defense Secretary Ash Carter announced the move during a visit to Baghdad, where he met US commanders, Iraqi Prime Minister Haider al-Abadi and Defence Minister Khaled al-Obeidi.
Most of the 560 additional troops will work out of Qayara air base, which Iraqi forces recaptured from DAESH and plan to use as a staging ground for an offensive to retake Mosul, Iraq's second biggest city.
Government forces have said they recovered the air base, about 60 km (40 miles) from the northern city, with air support from the US-led military coalition.
Carter told a gathering of US troops in Baghdad: "With these additional US forces I'm describing today, we'll bring unique capability to the campaign and provide critical support to the Iraqi forces at a key moment in the fight."
He said the new troops were "ready to come" and it would be a matter of "days and weeks, not months."
Prime Minister Haider al-Abadi has pledged to retake Mosul by the end of the year.
- Over 150,000 Iraqis killed
- 4,491 US troops killed
- Cost: $2.4 trillion by 2017
- No WMDs found
- Saddam Hussein executed
— The Int'l Spectator (@intlspectator) July 11, 2016
However, there is still debate in Washington about the timing of an offensive on Mosul.
Some US and allied military and intelligence officials warn that, aside from its elite counter terrorism force, the Iraqi military is not ready to take on DAESH militants in Mosul without significant assistance from the Kurdish peshmerga and Shiite militias.
Moreover, Baghdad and Arbil, the capital of the autonomous Kurdish region, do not seem to have agreed on a plan for Mosul, and any significant participation by Kurdish or Shiite forces in a Mosul campaign, one US official said, "would create a whole new set of problems that the Abadi government is incapable of managing, or even mitigating."
Separately, the Republican chairman of the House of Representatives Armed Services Committee has said that President Obama must ask Congress for additional funds to pay for the deployment of more troops to Iraq, as Congress and the White House debate defence spending amid mandatory budget cuts.
The latest US surge comes less than three months after Washington announced it would dispatch about 200 more soldiers to accompany Iraqi troops advancing towards Mosul. Carter told reporters ahead of Monday's trip that the United States would now help turn Qayara into a logistics hub.
The airfield is "one of the hubs from which ... Iraqi security forces, accompanied and advised by us as needed, will complete the southernmost envelopment of Mosul," he said.
Lt. Gen. Sean MacFarland, commander of the coalition against DAESH militants in Syria and Iraq, said the additional troops would fill a variety of roles.
"[They are] coming in to help expand the base at Qayara West airfield into a node that can support the Iraqi security forces as they move forward with the Mosul operation, and it’ll be an operational air base."
US forces have already visited Qayara to check on its condition, and advisors can offer specialised engineering support in Mosul, US officials said.
Earlier at the end of 2011 the US withdrew the last American troops from Iraq.
The deployment of 560 additional troops will bring the official number of US service members to 4,647 in war-torn Iraq. A decade ago around 130,000 US service members were stationed in the country.